Knock control fluids have been developed to mitigate various abnormal combustion events in engine cylinders. For example, various combinations of gasoline, ethanol, methanol, other alcohols, water, washer fluid, and other inert fluids may be direct injected into an engine cylinder in response to an indication of untimely detonation. Specifically, dual fuel systems may inject a primary fuel and secondary fluid including a knock control fluid such as water.
One example approach for injecting a knock control fluid to mitigate cylinder abnormal combustion is shown by Leone et al in US 2014/0202434. Therein, knock is addressed by injecting water into an engine cylinder to increase scavenging and then adjusting the water injection amount based exhaust oxygen content, knock, and additional engine operating parameters. However, the inventors herein have recognized that there may be potential issues associated with adjusting a water injection amount based on engine operating parameters alone. For example, the amount of water actually injected into the cylinder may be more or less than actually desired based on engine operating parameters. As a result, the engine may be exposed to knock (if the water injection amount is less than desired) or water may condense within the engine cylinder, thereby leading to dilution of oil with water (if the water injection amount is more than desired).
In one example, the issues described above may be addressed by a method for adjusting an amount of secondary fluid injected at an engine cylinder based on a first change in pumping current of an exhaust oxygen sensor between a first and second reference voltage when only fuel is injected into the engine cylinder and a second change in pumping current of the exhaust oxygen sensor between the first and second reference voltage when fuel and the secondary fluid are injected into the engine cylinder. As one example, the oxygen sensor is positioned in an exhaust passage downstream of the engine cylinder. Further, an engine controller may determine an estimated amount of secondary fluid injected at the engine cylinder based on a difference between the second output and the first output. As such, the engine controller may adjust the amount of secondary fluid injected at the engine cylinder based on the determined estimated amount secondary fluid injected. For example, the method may include increasing or decreasing the amount of secondary fluid injected at the engine cylinder responsive to the estimated amount of secondary fluid being a threshold amount different than a desired amount of secondary fluid injection. The desired amount of secondary fluid injection may be based on engine operating conditions such as knock, engine temperature, exhaust gas temperature, an amount of exhaust gas in the engine cylinder, and an oxygen content of exhaust gas, and the like. The desired amount of secondary fluid injected at the engine may also be based on a water content of the secondary fluid, as determined based on a change in pumping current of the oxygen sensor between the first and second reference voltage upon injecting a known fraction of secondary fluid at the engine cylinder. Thus, an actual amount of secondary fluid injected at the engine cylinder may be estimated based on outputs of the oxygen sensor during injection of just fuel and injection of fuel and secondary fluid. In this way, the control of secondary fluid injection may be increased, thereby reducing the occurrence of knock and also condensation within the engine cylinder.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.